Ancient Sharks

It would be a mistake to think that ancient and modern sharks are
more-or-less the same. Ancient sharks differed from modern sharks in several
important respects. Of course one would expect ancient and modern sharks to
share certain structural features: jaws, replaceable teeth, tooth-like
scales, paired fins, internal fertilization, and a cartilaginous skeleton.
It is these characteristics that define them as sharks. But, even in their
earliest days, these animals developed some remarkable variations on the
basic sharky theme.

Cladoselache

Cladoselache was something of an oddball among ancient sharks. A
four-foot (1.2-metre) long inhabitant of late Devonian seas (about 370
million years ago), it exhibited a strange combination of ancestral and
derived characteristics. Like many ancient sharks, Cladoselache had a
short, rounded snout, a mouth located at the front of the head (a mouth type
called "terminal"), long jaws attached to the cranium under the
snout and behind the eye, cladodont teeth, and a stout spine in front of
each dorsal fin. Yet it also had strong keels developed along the side of
the tail stalk and a crescent-shaped tail fin, with an upper lobe about the
same size as the lower (in most modern sharks, the tail is decidedly
top-heavy, with the upper lobe considerably longer than the lower). In these
posterior respects, Cladoselache resembles the modern mackerel sharks
of the family Lamnidae, a group which includes the white shark and its close
relatives, the makos and mackerel sharks. The combination of lateral keels
and crescentic tail fin is highly characteristic of fast-swimming fishes
such as tunas, billfishes, and mako sharks. Many paleontologists therefore
believe that Cladoselache was specialized as a high-speed
predator.
Remarkably well-preserved specimens from the Cleveland Shale of Ohio
support this notion.

Unlike most ancient and all modern sharks, Cladoselache swam the
seas virtually naked. Except for small, multi-cusped scales along the edges
its fins, in the mouth cavity, and around the eye, Cladoselache's
skin seems to have been almost devoid of the tooth-like scales that
characterize sharks as a group. Shark scales serve as more than simple armor
against injury, they strengthen the skin to provide firmer attachments for
swimming muscles, yet Cladoselache managed to make do almost without
them. Cladoselache's fin spines were odd, too. They were unusual in
being short and blade-like, composed of a porous bony material, and located
some distance anterior to the origin of each dorsal fin. These fin spines
may have been lighter and sturdier than the denser, more spike-like ones of
other sharks. These light-weight but stout fin spines may have reduced
swimming effort yet provided solid discouragement to would-be predators.

One aspect of Cladoselache's morphology is so unprecedented, it is
difficult to know what to make of it. Unlike any other shark, ancient or
modern, Cladoselache seems to have lacked claspers - the paired,
sausage-shaped organs that male sharks use to transfer sperm into the body
of female sharks. Other sharks had already developed claspers by the time of
Cladoselache's appearance. The xenacanths, for example - which
appeared some 50 million years before Cladoselache - had limb-like
claspers supported by skeletal elements which are sometimes preserved as
fossils. Diademodus, a contemporary of Cladoselache,
apparently also had well-developed claspers. It seems highly unlikely that
every known specimen of Cladoselache is female, so it is something of
a mystery how these sharks reproduced. Yet Cladoselache obviously
managed to procreate somehow, as its lineage survived for nearly 100 million
years. It may seem an unpleasant idea, but perhaps Cladoselache
achieved internal fertilization by partially extruding the rear part of its
cloaca and using that as the organ of sperm transfer. This is the method of
copulation used by most modern birds and a few modern amphibians and
reptiles - namely, the caecilians (which resemble legless salamanders) and
the lizard-like tuatara. We will probably never know for sure. Fossilized Cladoselache
don't kiss and tell.

About the same time Cladoselache first appeared, there evolved an
important group of sharks known as the ctenacanths. The ctenacanths shared
numerous conservative features with Cladoselache, but also developed
several more advanced ones. Like Cladoselache, the ctenacanths had
cladodont teeth, jaws attached to the skull at front and back, broad-based
pectoral fins, and a strong spine in front of each dorsal fin.. But unlike Cladoselache,
the pectorals of ctenacanths were supported at the base by three blocks of
cartilage - as in most modern sharks - allowing them greater flexibility.
Ctenacanths were also different in that their fin spines were long and
cylindrical, with characteristic longitudinal ridges and unique comb-like
rows of tubercles (hence their name). These spines were composed of a dense
enameloid material and deeply imbedded along the front margin of each dorsal
fin - as in modern spiny dogfishes (family Squalidae)
and bullhead sharks (Heterodontidae).

Ctenacanths

Finspine
of ctencanth ("comb-spine"), in cross-section (left) and
lateral (right) views, showing the characteristic ridges separating
series of tooth-like nodules.

Ctenacanths
are known almost entirely from abundant fossils of their distinctive fin
spines (body impressions or skeletal remains of these sharks are quite
rare). The best-known genus is Goodrichthyes, known from a 7.5-foot
(2.3-metre) specimen from early Carboniferous deposits in what is now
Scotland. Unfortunately, this specimen is contained in some 200 separate
pieces of rock, and is thus rather difficult to interpret. The genus Ctenacanthus
itself is represented by many species, almost all of them established on the
basis of fin spines. The ctenacanths appeared in the late Devonian (about
380 million years ago - slightly earlier than Cladoselache) and
persisted until the Permian, with a few hanging on into the Triassic (about
250 million years ago). But there is no doubt that their heyday - in terms
of diversity and abundance - was during the Carboniferous.